Planes, trains and a $5b promise for Victoria

Malcolm Turnbull's $5 billion promise to co-fund a Melbourne Airport rail link has been described as ambitious both in timing and planning by the state government, which is yet to get on board.

The prime minister's investment in the long-awaited airport passenger train service on Thursday was touted as a game changer for Victoria, but the idea relies on a funding partnership with Victoria's Labor government.

"The decision has to be made, leadership has to be shown, money has to be invested," Mr Turnbull told reporters at Melbourne Airport.

"The time for putting this in the too-hard basket is over."

There are four potential routes and Mr Turnbull hoped construction could start as early as 2020.

But the state government said the prime minister was getting ahead of himself.

"2020 might have been a bit too ambitious and I think he's already started to walk away from that," Public Transport Minister Jacinta Allan said.

The state government had already committed in the 2017 budget to further investigate an airport rail link and was considering models and routes, she added.

"For some months now Victoria has been leading the work on the preliminary business case ...(and) we will continue that work because it is important that we get this right."

"This is absolutely critical given the importance of this project to the city and the state, (and) the potential costs of the project."

Ms Allan said the four devised routes provided by Mr Turnbull were from a 2012 report by the state government and were outdated.

"Only one of those four potential route alignments could provide a connection with regional Victoria ... but connecting it through the growing areas of the north and west of the city makes sense."

Ms Allan hoped the two governments would be able to work together but added it was disappointing to read about the Commonwealth offer through the media, with a letter sent to Premier Daniel Andrews hours later.

"It's that sort of petty politics that's really frustrating. It's frustrating for those of us involved in politics and it's even more frustrating for people who just want to see this projects get done," she said.

Mr Turnbull defended his communication with the premier and request for a shared funding model.

"This is the new way the federal government works on infrastructure in cities. We want to work as partners," he said.

Opposition leader Bill Shorten backed the plan, despite two of the four proposed routes running through his electorate of Maribyrnong.

"What we now need to do is belt and brace it with some proper planning and detail, and of course some proper consultation," he said from Perth.

The rail project has been discussed for decades, initially by former Victorian Liberal premier Henry Bolte, who held office from the mid-1950s to 1972.

State opposition leader Matthew Guy said it was time to "just build" the link, now "real" money had been pledged by the federal government.